1. Field of the Invention
This invention relates to a process for the production of cellular polyurethanes using special amine-carbon dioxide adducts and to homogeneous mixtures of these adducts, optionally with monofluorotrichloromethane,
2. Description of the Prior Art
The process known among experts as "frothing" has been successfully used in the production of foams, particularly rigid foams, based on polyurethanes. In the frothing process, the reaction mixture is prepared in such a way that it actually enters the mold cavity in the form of a creamy foam. The advantage of this lies in the stable, continuous flow front which guarantees high surface quality. The need to include a blowing agent, which frequently causes swelling in the end product, is avoided.
This process is used in particular for certain refrigerator constructions and for large, heavily insulated refrigerators where the still foaming liquid reaction mixture is able to flow out from leaks in the cabinet which can necessitate expensive finishing work.
The frothing effect is obtained, for example, by addition of a low-boiling blowing agent, such as difluorodichloromethane or a mixture of monofluorotrichloromethane and difluorodichloromethane.
For example, in a standard commercial rigid foam of the type used for insulating refrigerating cabinets, if part of the monofluorotrichloromethane blowing agent (boiling point +23.7.degree. C.) is replaced by dichlorodifluoromethane (R 12, boiling point -29.8.degree. C.) and if the dichlorodifluoromethane is introduced into the polyol under pressure in liquid form, the reaction mixture emerges from the mixing head of the proportioning machine in creamy, prefoamed form through spontaneous evaporation of the R 12. In this form, the reaction mixture is unable to flow out from leaks in the refrigerator cabinet.
Unfortunately, the frothing process is by no means a universal solution to sealing problems. Once the foaming reaction mixture has filled the walls of the refrigerator cabinet, it is able to flow out from leaks, despite frothing, because of the foaming pressure. The flowing agents or blowing agent mixtures in question are difficult to process because their boiling points are below 0.degree. C. and because they have to be supplied in pressure cylinders. Proportioning the blowing agents through pumps of any type involves problems because the stoichiometric ratios involved have to be carefully maintained, which is very difficult when high-performance piston pumps are used due to leaks.
The frothing effect may also be obtained by the addition of large amounts of gas. For example, an expanded gas (such as air or nitrogen) is introduced into the polyol component in a quantity of about 0.4 to about 0.8 g per liter of polyol. Similar problems are involved here. Proportioning of the gas is carried out with flow meters and, accordingly, is inaccurate. The gas has to be very finely dispersed in the component because when the usual high pressure pumps are used for proportioning, it is only by intensive mixing that the required effects can be obtained. Further the mixture of gas and polyol component has to be kept under such high pressure that no proportioning errors are produced by the gas bubbles mixed in.
It has also been found that if the proportion of monofluorotrichloromethane which boils at 23.5.degree. C. is reduced and replaced by difluorodichloromethane or gas, the quality of the outer skin is seriously affected both in hardness and in thickness.
In addition, fluorinated hydrocarbons have been criticized by ecologists, for example by P. Fabian, Die Naturwissenschaften 67 (1980), pages 109 to 120, and C. R. Patrick, J. Fluorine Chem. 25 (1984), pages 7 to 15.
Accordingly, an object of the present invention is to provide a process for the production of rigid polyurethane foams by frothing which is not attended by any of the disadvantages of conventional processes and which
(a) may be carried out both in fixed industrial installations and also in situ using small mobile units,
(b) has physiological advantages over conventional processes using dichlorodifluoromethane and
(c) combines high operational reliability with improved handling.
According to the invention, these objects are achieved by the use of a special alkanolamine-CO.sub.2 adduct and/or alkanolamine -CO.sub.2 -water adduct in combination with a tertiary amine catalyst and optionally monofluorotrichloromethane in the form of a homogeneous mixture as blowing agent.
The use of carbon dioxide adducts of hydroxylamines in the production of polyurethanes is known. DE-OS No. 2,200,480 mentions the use of ethanolamine carbamate in a process for the production of a cellular PU-elastomer; however, this prior publication does not state whether the ethanolamine carbamate is solid or liquid. In addition, it is apparent from DE-PS No. 2,621,582 that carbamates, which are reaction products of primary or secondary amines containing 1 or 2 hydroxyl groups with carbon dioxide, are solid or may be obtained in solid form if they are prepared by introduction of carbon dioxide into alcohols or other solvents and cannot be homogeneously mixed in a single phase, even with a polyol. It is not apparent from this publication that the carbon dioxide adducts in question are liquid at room temperature or may be obtained in liquid form.
Liquid carbamates of compounds containing amino groups are described in DE-OS No. 1,570,571. However, these amines are limited to diamines or polyamines containing ether groups in which about 0.01 to 50% of the amino groups may be replaced by a hydroxyl group. However, the amines preferably used are high molecular weight amines with, in general, only a small content of basic nitrogen and amino groups often present as secondary nitrogen atoms which only allow a low uptake of carbon dioxide.
The use of the products according to DE-OS No. 2,200,480 and DE-PS No. 2,621,582 for the production of polyurethane foams is also known.
According to DE-OS No. 2,200,480, the amine carbamates generally loose their thermal stability and decompose at temperatures above about 35.degree. C. The same teaching can be found in DE-OS No. 1,570,571, in that the amines in question may only be converted into carbamates by reaction with CO.sub.2 at -40.degree. to +40.degree. C. and preferably at 0 to 20.degree. C. The carbamates prepared by way of example in this prior publication largely decompose at temperatures as low as room temperature or 30.degree. C..
According to EP No. 121,850, carbon dioxide adducts of monoamines or polyamines containing one or more hydroxyalkyl groups (the amino groups may be primary, secondary and/or tertiary amino groups and may be substituents or chain members in acyclic or cyclic aliphatic parent compounds) are obtained in liquid form and remain in liquid form without crystallizing provided they are prepared by the addition of amines containing hydroxyl groups to carbon dioxide and, optionally water. The adducts thus prepared are said to be homogeneously soluble in the polyols normally used for the production of polyurethanes, especially those having a functionality of greater than 2 and a molecular weight of 60 to 2000.
EP No. 121,850 also teaches that not only carbamates (i.e., ammonium salts of alkylcarbamate acids), but also carbonic acid salts (formed by carrying out the reaction of the polyamines containing hydroxyl groups with carbon dioxide in the presence of water) and mixtures thereof are liquid. The differences between carbamates as salts of carbamic acid and carbonates or hydrogen carbonates as salts of carbonic acid are well known and described for example, in A. Smola, Mitteilungen der Chemischen Forschungsinstituts der Wirtschaft Osterreichs 2, 38-40 and 43-45 (1948) (Chemical Abstracts 43 (1949), 6157 h).
However, it has been found that many of these products cannot be stored with trichlorofluoromethane or stored for prolonged periods in the polyol component without suffering a distinct reduction in reactivity.
Accordingly, a particular object of the present invention is to provide a process in which trichlorofluoromethane and an aminoalcohol-CO.sub.2 adduct can be stored together without any substantial change in reactivity and without the need for stabilizer or other known auxiliary.